If you use the typical stereo input to the soundcard then you can use a stereo Y and use both stereo channels, one for the optical and one for the audio. For a MIC input the three detectors can probably be wired in parallel, so you might need a Y or just a breakout board.

boyntonstu wrote:Audacity will show 4 blips if the sound travel time to the mic is longer than the projectile reaching the 2nd LED.

A really nice speed measuring experiment that I would like to see.

I would think the setup would always give four blips, unless you just managed to get the MIC at the perfect distance so the sound and 2nd photodetector got their signals at the same time. Even then, the two peaks are probably significantly different.

What exactly are you trying to learn with this experiment? Verifying and/or calibrating the optical chrony with the sound signal?

Place the PVC centered and lined up to the axis of your cannon barrel and shoot through it.

Works great for ammo up to perhaps 1/2"D. Personally though, I don't see a lot of need for the LEDs unless you plan on shooting in the dark. If you mount the detector on the muzzle then you don't have to worry so much about the alignment between the detector and the round's line of flight. The holder for the transistors can be made from a chopped up piece of the same pipe the barrel is made out of. The barrel is green, the transistor holder gray and the holder just clips onto the barrel. The holder is automatically offset from the ammos line of flight by one pipe wall thickness.